Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

libcef/browser/extensions/extension_web_contents_observer.cc

@@ -15,7 +15,9 @@ namespace extensions {
 
 CefExtensionWebContentsObserver::CefExtensionWebContentsObserver(
     content::WebContents* web_contents)
-    : ExtensionWebContentsObserver(web_contents) {}
+    : ExtensionWebContentsObserver(web_contents),
+      script_executor_(
+          new ScriptExecutor(web_contents, &script_execution_observers_)) {}
 
 CefExtensionWebContentsObserver::~CefExtensionWebContentsObserver() {}